Yes! I got it working! Thanks heaps Funky, and especially you faceless!
Here's a complete process of what I did for those who have trouble working out the ins & outs of DS emu:
1. Ensure you have these 2 programs:
* Rein (Rein15)
* Supercard Converter (scconv01)
2. Drag REIN.nds onto scconv
3. Put the newly created REIN.SC.nds file onto the SD card
4. Start the DS in DS mode with the Supercard & SD together in the GBA slot,
 and the Superpass and DS game (eg. MPH-FH demo) in the DS slot
5. Locate and open up the REIN.SC.nds file
6. It will say a Superpass is inserted and that it can't read valid save files..
 .. but just press A anyway to bring up the Rein menu
7. Now remove the Superpass and insert the original DS card which you want to work with
8. Select BACKUP, press A, and again to confirm the process
9. Once completed, press A to return back to the Rein menu screen
10. Repeat procedure with a new DS card as necessary
...
Easy as. Cheers!